Raise ImportError for missing 'hstcal' binaries

[sean-lockwood]

Check the $PATH for hstcal binaries (cs0.e) when importing stistools. Raise an ImportError if this is not found. Skips this check on RTD.

[sean-lockwood]

@stscirij -
Any problem raising an error on import like this?

[stscirij]

I think this seems reasonable, although it probably won't work under Windows :)

[sean-lockwood]

Looks like hstcal doesn't support Windows anyway:

[sean-lockwood]

Some of the stistools routines don't require an hstcal binary. Perhaps we should more precisely where to put this check?

[sean-lockwood]

Less invasive alternatives:

1. Issue a warning instead
2. Move check into relevant functions:

basic2d.basic2d
calstis.calstis
ocrreject.ocrreject
wavecal.wavecal
wavecal.runBasic2d
wavecal.runCs11
wavecal.runWavecal
wavecal.runX2d
wavecal.runCs12
x1d.x1d
x2d.x2d

(`*.main` functions are not used, so probably don't need to be updated.)